The present invention relates to an elementary cell structure for programmable time-continuous analog filters and in particular for read/write operations on magnetic supports.
Specifically but not exclusively the present invention relates to a cell comprising an amplifier stage inserted between a first reference supply voltage and a second reference voltage having a first and a second input terminals and an output terminal.
The present invention also relates to a transverse analog filter and a delay chain provided by means of a series of cells of the above type.
As known, in the specific field of application of the present invention there is a need to provide electronic circuits designed for processing analog signals during data read and write phases from and to a magnetic support, e.g. a hard disk of an electronic processor. The signals must be processed in an appropriate manner to avoid data reading and memorization errors.
There is for example a technique known as Partial Response Signaling With Maximum Likelihood Sequence Detection (PRML) which provides only partial sampling of the frequency band of the input signal and the successive search for the highest correspondence possible between the output signal and the input signal sample.
Normally in a PRML architecture an analog signal picked up e.g. by a read/write head of the magnetic support is processed and reconstructed in digital format by means of an equalizing device. Specifically the analog signal is first subjected to an analog equalization by means of a variable-gain input amplifier and successively processed through a time-continuous low-pass analog filter. The majority of analog filters can be manufactured starting from low-pass elementary cells.
The output of the analog filter is successively sampled through an analog/digital converter and further processed through a transverse digital filter, e.g. the Finite Impulse Response (FIR) type which has a transfer function of the following type:
y(t)=a0+a1D1+. . . +anDnxe2x80x83xe2x80x83(1)
where:
a0, a1, . . . , an are coefficients chosen according to the type of use of the filter, and
D1, . . . , Dn is the input signal delayed n times the unit delay.
Normally each of the elementary cells making up a FIR filter has a transfer function to supply at the output the input signal delayed according to a predetermined value, called unit delay.
The delay with which the input signal appears at the output is a function of the signal frequency. More specifically the delay is the derivative in relation to the frequency with changed sign of the transfer function phase between the input and the output.
The prior art proposes to provide the FIR filters in a digital or sampled time manner. These solutions are however affected by an error due to the quantization of the analog signal which must be converted into a digital signal and by another error introduced by the sampling of the signal and commonly called xe2x80x9caliasingxe2x80x9d phenomenon.
The presence of these errors limits the use of digital or sampled time FIR filters in many applications as for example those for magnetic support read/write devices.
The technical problem underlying the present invention is to conceive an elementary cell structure of the analog type to provide time-continuous filters having structural and functional characteristics allowing replacing digital filters which still limit the use of the PRML technique in read/write devices provided in accordance with the prior art.
Another purpose of the present invention is to conceive a delay line achieved with a chain of elementary cells allowing introduction in the filtering phase of a delay of any predetermined value without thereby distorting the input signal either in phase or in amplitude.
The solution idea underlying the present invention is to provide a symmetrical structure of the time-continuous analog type comprising two identical half-cells provided starting from a transconductance amplifier stage.
The problem is solved by a transverse analog filter comprising a plurality of elementary cells in accordance with claim 1 and following.
The characteristics and advantages of the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings.